1. Field of the Invention
This invention relates to a reflecting mirror optical system, and in particular to a reflecting reduction-projecting optical apparatus for reduction-projecting the pattern of a mask (negative) onto a wafer having photoresist applied thereto in the manufacture of a semiconductive integrated circuit.
2. Related Background Art
Exposure apparatuses for the manufacture of semiconductors such as steppers have heretofore used near-ultraviolet light. In recent years, excimer laser and far-ultraviolet light of shorter wavelength have also been used, and with the tendency of integrated circuit patterns toward minuteness, exposure apparatuses of higher resolving power have been desired. The resolving power is determined by exposure wavelength and the numerical aperture of the optical system, and the shorter is the exposure wavelength and the greater is the numerical aperture, exposure transfer of the more minute pattern becomes possible, but it is not advisable to make the numerical aperture great because it involves difficulties in optical design and results in a shallow depth of focus and thus, attempts to shorten the exposure wavelength have been made.
For the manufacture of a semiconductor element comprising a very minute pattern and having a memory capacity of the order of 256 Mbit or more, such as super-LSI, it becomes necessary to exposure-transfer a line width (pattern) of 0.25 .mu.m or less and for this purpose, it becomes indispensable to use soft X-rays or X-rays.
Exposure apparatuses using X-rays have already been developed, but those so far developed are of the so-called proximity type in which the shadow picture of a mask is transferred to a wafer, and requires exacting pattern accuracy of the mask and involves difficulties in the manufacture of the mask. Therefore, a reflecting reduction optical system which reduces a mask image and projects and transfers it onto the surface of a wafer is regarded as promising.
As a reflecting reduction optical system, there is one by Shafer et al. disclosed, for example, in U.S. Pat. No. 4,747,678. This is basically comprised of three concave reflecting mirrors and a convex reflecting mirror, but due to aberrations (coma and spherical aberration) caused by the convex mirror for effecting reduction, it is insufficient to obtain such a degree of resolving power that is used for the manufacture of a semiconductor element. A construction in which a refracting member is added to correct these aberrations has also been proposed at the same time and by the introduction of the refracting member, a good formed image is obtained in an arcuate field of view for the first time. However, the refracting member does not stand practical use in a soft X-ray or X-ray area.
Reflecting reduction optical systems by Suzuki et al. have also been proposed as disclosed in Japanese Laid-Open Patent Application No. 63-311315, etc., but these introduces therein an aspherical reflecting mirror for the correction of aberrations and the manufacture of such optical systems becomes remarkably difficult.
Accordingly, the reflecting surface which can be practically used is only a spherical surface. The Schwarzschild type has long been known as a reflecting optical system comprised of only a spherical surface. As shown in FIG. 8 of the accompanying drawings, this is an optical system comprised of two concentric spherical reflecting mirrors, i.e., a concave mirror 1 and a convex mirror 2, and is a very excellent optical system. However, its only disadvantage is that it is a concentric optical system and therefore its object plane and image plane are spherically curved about the centers of sphere of the reflecting mirrors. If the object plane is made into a flat plane, the image plane will be further curved. Therefore, this optical system could not be used in a semiconductor manufacturing apparatus which required a wide exposure area.